A356456 Numbers k not divisible by 3 such that 2*k^2 is not in A014567.

10, 28, 40, 50, 70, 77, 110, 130, 133, 136, 140, 154, 155, 160, 161, 170, 176, 190, 196, 200, 209, 224, 230, 250, 259, 266, 275, 280, 290, 308, 310, 322, 350, 364, 370, 371, 377, 385, 410, 416, 418, 430, 440, 469, 470, 476, 490, 496, 518, 520, 530, 532, 539, 550, 553, 590

Offset

1,1

Comments

Numbers k not divisible by 3 such that k and sigma(2*k^2) are not coprime, sigma = A000203.

Note that if m is twice a square, then sigma(m) is divisible by 3 (cf. A065766).

Links

Jianing Song, Table of n, a(n) for n = 1..11669 (all terms <= 100000)

Example

28 is a term since 28 and sigma(2*28^2) = 3591 have a common factor 7, and 28 is not divisible by 3.

Prog

(PARI) isA356456(n) = (n%3) && gcd(n, sigma(2*n^2))>1

Crossrefs

Cf. A014567, A000203, A065766.

Equals A356453 \ A008585.

Sequence in context: A022422 A113962 A031105 * A302529 A054501 A112353

Adjacent sequences: A356453 A356454 A356455 * A356457 A356458 A356459

Keyword

nonn,easy

Author

Jianing Song, Aug 07 2022

Status

approved